Health Care

Semi-identical twins discovered in Queensland

Semi-identical twins discovered in Queensland”

Semi-identical twins are extremely rare-the last reported case was in 2007.

The boy and girl, who are now four, from Brisbane, Australia, are identical on their mother's side but only share a proportion of their father's DNA.

Each sperm cell contains half the father's DNA, but it is not identical from sperm to sperm because each man is a mixture of the genetic material from his parents, and each time a slightly different assortment of that full DNA set gets divided to go into a sperm.

Michael Gabbett, a geneticist at Queensland University of Technology, and one of his co-authors, Nicholas Fisk, an obstetrician and deputy vice-chancellor of research at the University of New South Wales, oversaw the twins' fetal care back in 2014 at the Royal Brisbane and Women's Hospital. There is another type of twin you may have never heard of because of the rarity, they are called semi-identical twins.

Genetic testing revealed that the children were identical, or monozygotic, on their maternal side, but shared 78 percent of their paternal genome, "which makes them genetically in between monozygotic and dizygotic; they are sesquizygotic", the authors write in their paper.

However, they eventually realised that the twins were different sexes - meaning that it would have been impossible for them to be identical.

"It is likely the mother's egg was fertilised simultaneously by two of the father's sperm before dividing", Fisk said in the statement.

In addition, the girl and boy each have male and female sex chromosomes - in other words, each twin has some cells that carry an XX pair (female) and some that carry an XY pair (male).

At first, doctors wrongly assumed the foetuses were identical twins because an ultrasound taken early in the pregnancy showed them sharing the same placenta.

In this case, the embryo reportedly split in two with each ending up with the right number of chromosomes. Her ultrasounds at six weeks indicated she was expecting identical twins, but the 14-week scan "showed the twins were male and female, which is not possible for identical twins", Fisk said.

It is only the world's second known case of semi-identical twins.

This type of twinning occurs when more than one egg is released from the ovary at ovulation (normally just a single egg is selected and released), and both of the eggs are fertilised by different sperm. Researchers from the University of New South Wales and the Queensland University of Technology combed through almost 1,000 cases of twins to confirm their findings.

The discovery was actually made before the twins were born.

They were brought to the attention of doctors because one had "ambiguous" genitalia.

The team analyzed genetic data from 968 fraternal twins as well as from other studies but found no other cases.

Shortly after birth, the girl also developed a clot that cut off blood supply to her arm, which resulted in the limb being amputated. The twins are healthy and have hit all developmental milestones.



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